This application is based upon and claims priority of Japanese Patent Application No. 2002-173135, filed on Jun. 13, 2002, the contents being incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method of manufacturing a ferroelectric capacitor constituted by sandwiching a ferroelectric film between a pair of electrodes.
2. Description of the Prior Art
A ferroelectric capacitor utilizes spontaneous polarization phenomenon of a ferroelectric film. A ferroelectric random access memory (FeRAM) including this ferroelectric capacitor is a non-volatile memory which can retain data without a power. The FeRAM has advantages of low voltage requirement upon data writing, and capabilities of high-speed operation and high-frequency data rewriting.
The ferroelectric capacitor is constituted by sandwiching a ferroelectric thin film such as PZT (Pb(Zr,Ti)O3) between a pair of electrodes. In the following, description will be made regarding a conventional method of manufacturing a ferroelectric capacitor with reference to FIGS. 1A and 1B.
First, as shown in FIG. 1A, an element such as a transistor (not shown) is formed on a semiconductor substrate 60, and surfaces of the element and the semiconductor substrate 60 are covered with an interlayer insulation film 61. Then, a conductive film 62 which constitutes a lower electrode is formed on the interlayer insulation film 61 by using a conductive material such as platinum (Pt).
Subsequently, a PZT film 63 is formed on the conductive film 62. Sputtering method is generally applied as a method of forming the PZT film 63. However, spin coating method, sol-gel method, metal organic chemical vapor deposition (MOCVD) method or the like may be applied as appropriate.
Thereafter, the PZT film 63 is subjected to crystallization treatment (annealing). In this crystallization treatment, the PZT film 63 is subjected to heating at the temperature of 600xc2x0 C. for several tens of seconds, for example.
Next, a conductive film 64 which constitutes an upper electrode is formed on the PZT film 63 by using a conductive material such as Pt.
Subsequently, the conductive film 64, the PZT film 63, and the conductive film 62 are patterned into given shapes by photolithography method, whereby an upper electrode 64a, a ferroelectric film 63a, and a lower electrode 62a are formed as shown in FIG. 1B. In this way, a ferroelectric capacitor is completed.
The inventors of the present invention consider that the above-described conventional method of manufacturing a ferroelectric capacitor has following problems.
FIG. 2 is a scanning electron microscope (SEM) image showing a surface of the PZT film formed by the conventional method. As shown in FIG. 2, the surface of the PZT film formed by the conventional method has relatively high unevenness, and there are tiny pores in grain boundaries. For this reason, problems such as an increase in a leak current occur particularly when the PZT film is thin (about 150 nm or below). In such a case, the ferroelectric capacitor hardly meets required electric characteristics.
In consideration of the foregoing problems, it is an object of the present invention to provide a method of manufacturing a ferroelectric capacitor which enables to manufacture a ferroelectric capacitor with low unevenness on the ferroelectric film surface and with excellent electric characteristics.
The forgoing problems will be solved by a method of manufacturing a ferroelectric capacitor, which includes the steps of: forming a lower electrode on a semiconductor substrate via an insulation film; forming a ferroelectric film on the lower electrode; subjecting the ferroelectric film to crystallization treatment; forming a sintering assistance film on the ferroelectric film; subjecting the ferroelectric film to sintering treatment; and forming an upper electrode on the ferroelectric film.
Moreover, the foregoing problems will be also solved by a method of manufacturing a ferroelectric capacitor, which includes the steps of: forming a lower electrode on a semiconductor substrate via an insulation film; forming a ferroelectric film on the lower electrode; and forming an upper electrode on the ferroelectric film. Here, the ferroelectric film is formed by repeating a plurality of times the steps of forming a ferroelectric film, forming a sintering assistance film on the ferroelectric film, and performing sintering treatment.
Furthermore, the foregoing problems will be also solved by a method of manufacturing a ferroelectric capacitor, which includes the steps of: forming a lower electrode on a semiconductor substrate via an insulation film; forming a ferroelectric film on the lower electrode; and forming an upper electrode on the ferroelectric film. Here, the ferroelectric film is formed by repeating a plurality of times the steps of forming a ferroelectric film and forming a sintering assistance film on the ferroelectric film, and then by performing sintering treatment.
At the time of formation by sputtering method or the like, the ferroelectric film is amorphous and thereby does not have a ferroelectric characteristic. Therefore, it is necessary to subject the ferroelectric film to heat treatment to crystallize the ferroelectric material. However, only heat treatment cannot achieve sufficient crystallinity. Therefore, relatively high unevenness are developed on the ferroelectric film surface as described above, and the unevenness causes degradation in the electric characteristics.
To improve the crystallinity, it is also conceivable to perform the heat treatment at a higher temperature. However, even the heat treatment at the higher temperature cannot actually reduce the unevenness on the ferroelectric film surface.
The inventors of the present invention have performed various experiments and investigations for eliminating such the unevenness on the ferroelectric film surface. As a result, the inventors have found that the crystallinity is improved so as to even the ferroelectric film surface by firstly coating the sintering assistance on the ferroelectric film surface after the crystallization treatment and by secondly performing the heat treatment. The present invention has been made based on experimental results relevant thereto. Silicate can be used as the sintering assistance, for example.
Moreover, the sintering assistance can be mixed with the ferroelectric material, and the heat treatment can be performed after coating the mixture on the ferroelectric film surface.
When the ferroelectric film is formed by sol-gel method, a plurality of ferroelectric layers is laminated. In this case, it is possible to form the ferroelectric film with excellent electric characteristics by using the sintering assistance in every formation of the ferroelectric layer and then by sintering. Here, it is not always requisite to perform sintering treatment in every formation of the ferroelectric film, but is also possible to perform the sintering treatment just once after alternately laminating the ferroelectric layers and the sintering assistance films.